Large diameter pipe strings are frequently made up under circumstances demanding that successive joints of pipe be connected by a coupling which, on the one hand, is capable of withstanding very large axial loads in both compression and tension yet, on the other hand, can be made up and separated very quickly by simple manipulations. The circumstances encountered during installation of casing and riser strings from an offshore drilling vessel or platform are typical. A typical conductor casing and riser string can be made up of pipe joints which are up to 60 ft. in length and 36 in. in outer diameter, the length of the combined string ranging from hundreds of feet to thousands of feet. Large tensile loads on such a string arise simply from the weight of the pipe, and since the string frequently must be driven into the well hole, very heavy compression loads and complex bending loads occur. Further, the riser of an underwater well must be maintained under tension, as by use of buoyancy devices or mechanical tensioning devices and heavy tension loads are therefore applied and, due to wave action at the vessel or platform, may vary cyclically.
Though such large diameter pipe strings can be made up with threaded couplings, the strength requirements make such couplings expensive, and making up a threaded coupling requires excessive time and labor, particularly under the circumstances encountered in offshore operations. Accordingly, workers in the art have proposed to use unthreaded quick make-and-break couplings depending on interengaged elements in the nature of "no-lead" thread segments as shown, for example, in U.S. Pat. No. 3,974,875, issued Aug. 17, 1976, to Herd et al. Other prior-art devices (as disclosed below) have used relatively heavy lugs to retain the male and female members of the coupling engaged. In prior-art devices of both types, a male connector member is welded to one end of each joint of pipe and a female member is welded to the other end, two joints of pipe being connected, for example, by lowering the male connector member of one joint into the upwardly directed female connector member of the joint last installed in the string, the lowered pipe then being rotated through part of one revolution to bring the segments or lugs of the male member into engagement beneath the segments or lugs of the female member.
In an effort to avoid occurrence of radial loads which tend to separate the two coupling members, prior-art workers have employed segments or lugs the engaged surfaces of which lie in planes at right angles to the longitudinal axis of the coupling. Surprisingly, however, it has been discovered that, while that prior-art practice is theoretically desirable, it actually increases the chances for failure of the coupling under conditions of actual use, especially when the coupling employs relatively small "no-lead" thread segments. While such couplings have achieved commercial acceptance, there has been a continuing need for improvement.